Anodic Voltage Dependence of Ti-6Al-4V Substrates and Hydroxyapatite Coating

Ti-6Al-4V alloys were anodized in a solution containing 0.15 M HF and 2 M H₃PO₄ for 30 min under different voltages and then coated with hydroxyapatite (HA) by hydrothermal-electrochemical deposition. The effects of anodizing voltage on the morphology and bioactivity of the HA coating and on the bonding strength between the HA coating and the anodized substrates were investigated. Results indicated that highly ordered amorphous TiO₂ nanotube arrays formed on the Ti-6Al-4V surface after anodic oxidation. The pore size of the nanotube increased up to approximately 100 nm with increasing anodic voltage until 25 V. The nanotube was damaged at anodic voltages above 25 V. The crystal structure of TiO₂ changed from amorphous to anatase when the anodized substrates were heated at 450 °C for 3 h. The contact angle between the Ti-6Al-4V surfaces and the simulated body fluid evidently decreased after anodic oxidation. The roughness increased with increasing anodic voltage, and Ra reached about 0.56 μm under 25 V. The HA coating exhibited layered growth. The deposition of rod-like HA crystals as well as the crystallinity of the HA coating initially increased and then decreased with the further increase of the anodic volatage. The degree of crystallinity reached the maximum of approximately 73% at 25 V. The bonding strength between the coating and the anodized substrates increased and then slightly decreased with increasing voltage. The bonding strength was about 20.0 MPa when titanium substrate was anodized under 25 V. The results of simulated body fluid immersing experiments suggest that the HA coating exhibits promising bioactivity.